The present invention relates to electrical filters and, more particularly, to bandpass waveguide filters for use at microwave frequencies.
The use of microwave filters is well known in the prior art and includes filters operable in the gigahertz frequency ranges for both bandpass and bandstop filter applications. Waveguide filters have been successfully used at the upper ranges of the GHz frequencies and other filter configurations, including dielectric resonators, have been employed for use at various frequencies. Examples of such filters and their characteristics are discussed in the article entitled "Application of Dielectric Resonators in Microwave Components" by Plourde and Chung-Li Ren in IEEE Transactions on Microwave Theory and Techniques, Volume MTT-29, No. 8, Aug. 1981, pp. 754-770, and in the book entitled Microwave Filters, Impedance-matching Networks, and Coupling Structures by Matthew, Young and Jones, McGraw Hill, 1964, pp. 450-459.
The above articles discuss the types of filter structures that may be constructed using dielectric materials and disclose techniques for controlling tuning and operation in any given frequency range. As is apparent, use of such dielectric materials acting as resonators for forming microwave filters, requires the selection of materials having parameters which fall within certain limits. More specifically, the quality factor Q should typically be selected to be approximately 10,000 or greater; the dielectric constant (.epsilon.) should be selected to have a value greater than 35; and the temperature coefficient of resonant frequency T.sub.f should be chosen to be less than 10 ppm/.degree. C. Even when constructed in accordance with the techniques disclosed in the above articles, however there are still limitations which restrict the use of such filters in particular applications.
By way of example, for a dielectric material such as titanium dioxide (rutile phase) which has a Q of 10,000 at 4 GHz and an .epsilon. of 100, the value for T.sub.f is 400 ppm/.degree. C. which is too high to be useful for practical applications. As explained in the article, for an ambient temperature change of 50.degree. C., the filter frequency of a 4 GHz titanium dioxide resonantor will shift by 80 MHz, which is unacceptable since the bandwidth required for many filters is less than that change. While different materials having less acceptable values for Q and .epsilon. may be used in order to obtain a better value of T.sub.f, the prior art filter configurations have not been capable of providing versatile filters in the 1-5 GHz frequency range which are simple and reliable in operation.
While waveguide filters capable of achieving acceptable results are known in the art, the size of the structure is usually so large at the 1-5 GHz range that it is unacceptable for most applications. For example, an air waveguide filter structure for use at 1-5 GHz would most likely have a length of about 2-6 feet in most applications. In current sophisticated electronic and avionics systems, such size and weight restrictions are unacceptable and prohibit effective use of such filters. There is therefore a need for more simplified structures which can be produced at less cost and are more versatile in operation.
Accordingly, the present invention has been developed to overcome the shortcomings of the above known and similar techniques and to provide a low frequency microwave bandpass filter useful in the 1-5 GHz range.